drm/udl: fix Bpp calculation in dumb_create()

Probably a typo.. we obviously need "(bpp + 7) / 8" instead of
"(bpp + 1) / 8". Unlikely to be hit in any sane code, but lets be safe.
Use DIV_ROUND_UP() to avoid the problem entirely and make the core more
readable.

Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>